This invention relates generally to apparatus for removing particulate contaminants from a gaseous stream, and more particularly to a self-cleaning electro-inertial precipitator unit in which particles charged by ions are induced to migrate toward a downwardly-flowing liquid film formed on the inner surface of a collector tube, the migration resulting from the combined action of electrostatic and centrifugal forces whereby the stream may be purified in the course of its passage through a relatively short collector tube.
Electrostatic precipitators separate contaminating particles or droplets of a semi-solid or solid nature from a gaseous stream. Such precipitators are especially helpful in removing finer particles (less than 40.mu.) which cannot be extracted by conventional filters or other particle separators. In one known form of electrostatic precipitator of the dry type, the gases to be purified are conveyed through a collector tube where the particles are charged with ions in an electrostatic field, the charged particles migrating toward the inner surface of the collector tube having an opposite charge, thereby separating the particles from the gas flowing through the tube. With continued operation of a dry precipitator, the particles accumulate on the wall of the collector tube and it becomes necessary, therefore, at fairly frequent intervals, to shut down the precipitator in order to permit removal of the agglomerated particles.
With a wet-wall precipitator of the type disclosed, for example, in the deSeversky Pat. No. 3,716,966, a uniform film of downwardly flowing water is formed on the inner surface of the collector tube, the film serving to continuously flush away the contaminants, thereby obviating the need to interrupt the operation of the precipitator.
The use of centrifugal separators or cyclonic collectors for separating dust particles and other particulate contaminants of 25.mu. or larger from a gaseous stream is well known. In order, therefore, to effectively remove both large and small particles from a gaseous stream, one may first feed the gaseous stream through a cyclonic collector or inertial dust separator stage to extract the large particles from the stream and then feed the partially purified stream through an electrostatic precipitator stage to extract the fine particles therefrom as well as those larger particles not extracted in the preceding stage.
Thus Pat. No. 3,315,445 to deSeversky discloses a pollution control system in which gas scrubber and wet electrostatic precipitator stages are intercoupled in cascade relation so as to remove the full spectrum of contaminants from the stream. The practical drawback to the deSeversky arrangement, apart from the relatively high cost of providing both a gas scrubber and a wet electrostatic precipitator, is that these two units occupy a substantial amount of space. This creates installation difficulties in those installations where space is at a premium.
In the above-identified related patent applications, there is disclosed an electro-inertial wet-wall precipitator unit in which both fine and coarse particles are extracted from a contaminated gaseous stream by the combined action of centrifugal and electrostatic forces. The advantage of the apparatus disclosed in the prior applications is that it carries out in a single compact, integrated unit, functions heretofore requiring at least two units.
In the electro-inertial precipitator disclosed in the prior applications, the gaseous stream to be purified is fed at high velocity tangentially into an upper inlet section of a vertical collector tube to impart a cyclonic or swirling motion thereto, thereby causing the gas to flow in a helical path down the tube along a downwardly-flowing water film to impose an inertial force which imparts a swirling motion thereto serving to maintain the film against the tube surface.
Supported coaxially within the collector tube is a discharge electrode, a high voltage being impressed between the electrode and the water film to create an electrostatic field therebetween the ions generated by the discharge electrode charging the particles carried by the gas. The centrifugal force created by the swirling motion of the gas induces the particles conveyed thereby to migrate toward the water film. This migration is further promoted by the action of the electrostatic field which causes the charged particles to travel toward the oppositely-charged water film. As a consequence, both coarse and fine particles are extracted from the gas and captured by the water film which washes the particles into the sump below the outlet section of the tube.
It has been found that when an electro-inertial wet-wall precipitator of the prior type is used to extract fine, low-density dust from a gaseous stream, such as sub-200 mesh grain dust in concentrations typically encountered in dusthandling systems in grain elevators, the operation of the unit is impaired by the nature and concentration of such dust.
To begin with, the grain dust does not wash cleanly from the inner surface of the tube, for spiral dust streaks tend to develop thereon, even though a normal water flow rate of about 0.5 gallons per minute per 1000 C.F.M. of gas is sufficient to keep the tube clean with low concentrations of dust such as cotton dust encountered in textile mills. Once these dust streaks are developed, even an above-normal increase in water flow rate will fail to flush the streaks away. While these streaks could be prevented from forming by setting the flow rate at start up to an above-normal level, this increase in flow rate eventually leads to water entrainment in the gaseous stream and requires more water processing and greater power to pump the water.
It has also been found that grain dust tends to form a cake at the upstream side of the water inlet slot in the precipitator tube, this cake slightly overlapping the slot at various points, thereby impeding the water flow and disturbing the uniformity of the water film. Moreover, these cakes occasionally break off and deposit on the wet wall at sites where they are difficult to wash away, such occurrences sometimes giving rise to arcing. In addition to dust streaks and dust cakes, dust deposits are formed in other regions of the precipitator structure which act to foul the unit and interfere with its proper operation.
Another problem encountered in wet-wall precipitator units in which a discharge electrode wire is extended between electrical insulating rods is that the rod which is exposed to the incoming contaminated gas stream will in time acquire a deposit of conductive particles thereon when the gas stream is the effluent of a welding process or other industrial operation which discharges more or less electrically-conductive particles into the atmosphere. This conductive deposit on the insulating rod degrades its insulating properties and may result in an electrical breakdown.
Yet another problem encountered in wet-wall precipitator units, particularly those which make use of large diameter collector tubes which operate at exceptionally high voltages exceeding 100 KV, is arcing as a result of water or other liquid projecting from the water inlet into the air gap between the discharge electrode and the inner surface of the collector tube. Ideally, water from the inlet should flow downwardly therefrom against the inner surface of the collector tube to create a water film thereon; but in practice, the configuration of the inlet and the velocity of water flow are such as to cause the water to somewhat shoot out of the inlet into the air gap to provide a conductive path in the air gap giving rise to arcing.